Suigetsu was seen to be somehow contained within a water-tank of sorts, obviously by Orochimaru.
So Oroshimaru > Suigetsu.
But, as far as we know, Sauke killed Orochimaru -
Orochimaru < Sasuke
Thus, Sasuke > Suigetsu.
Of course, that doesn't have to be true. Suigetsu could be there because he wanted to - he could've been working with Orochimaru. But it seems obvious that he has some sort of friendship with Sasuke, or at least acts like he's indebted to him for releasing him from the tank.
In other words, I can't see Suigetsu attacking Sasuke any time soon. The future, however...
I can already see Suigetsu doing lots of water jutsu. I'm betting he'll have his drink close at all times - he probably needs to take a lot of water just in order to survive.